The present invention relates to an electrically controllable filter device. Such devices find their application within optics, but also within microwave technology etc. In the case of optical filters in waveguide form these are mostly fabricated in such a way that the grating is obtained in a boundary surface between two dielectrics close to the waveguide, e.g. through etching. Another way to obtain a filter structure is to make an electrode structure form a grating. It is in many cases desirable to be able to tune the grating as well as to control its strength which means that the effective refractive index of the waveguide mode has to be varied etc.
Devices as referred to above are known. These devices, however, work in such a way that a grating is introduced through the mere existence of the electrodes. The grating effect thereof is in some cases even stronger than that which can be controlled via electrical feeding. A consequence hereof is that the grating cannot be "shut off". This means consequently also that the strength cannot be controlled.
Through the European patent application EP-A-0 267 667 a DFB-laser is known which comprises an etched grating. With the use of a so called divided electrode structure the current injection can be controlled so that the period can be modified and it is thus possible to have an influence on those parts of the grating which are to be the predominanting. A grating is, however, always present and it cannot be shut off. U.S. Pat. No. 4,008,947 shows an electro-optic switch or a modulator wherein a grating can be created through the application of a voltage. However, this grating is not tunable and a change in periodicity cannot be achieved. So called DFB-, and DBR- lasers normally use an optical filter structure in a waveguide form wherein the grating for example is etched in a boundary surface between two dielectrics close to the waveguide as mentioned above. The grating thereby acts as when the grating period is .lambda./2. If it, however, is desired to tune the grating a change in effective refractive index for the waveguide mode is required which gives rise to a number of problems. It also involves substantial difficulties to control the strength of the grating. However, a strength control of the grating could be achieved through the use of a periodical variation in refractive index (real and/or imaginary part) induced in an electrical way through a periodical electrode configuration instead of an etched grating. The refractive index may e.g. be influenced through the electrooptic effect or through injection of charge carriers. However, as mentioned above, the electrodes as such give rise to a grating effect which particularly is so strong that it exceeds the desired grating effect which can be controlled electrically and it is impossible to completely eliminate the grating effect or to "shut off" the grating.